1. Field of the Invention
The present invention relates to a semiconductor ceramic composition having positive temperature characteristics and which can withstand remarkably high voltages.
2. Description of the Prior Art
In general, it is known that if ceramic compositions could be prepared having various temperatures Tc (temperature for resulting positive characteristic) and various temperature coefficients for resistance, the fields of use for ceramic compositions would be enlarged. Accordingly, it has been proposed to substitute strontium for a part of the barium of a barium titanate-type semiconductor, or to substitute tin for part of the titanium so as to decrease a Curie point (temperature for resulting positive temperature characteristics) of the barium titanate which is 120.degree. C. It has been proposed in German Pat. No. 929,350 to substitute lead for part of the barium so as to increase the temperature for initiating the resistance increase to higher than 120.degree. C. It has been proposed in Japanese Patent Publication No. 17784/1964 to substitute strontium and calcium for part of the barium in barium titanate-type semiconductors so as to decrease the Curie point, whereby the Curie point (temperature for resulting positive temperature characteristics) is about room temperature and the variation of resistance caused by variation of voltage is decreased.
Recently, there have been remarkable improvements in the utilization of barium titanate-type semiconductor ceramic compositions as heating elements. It was necessary to increase the Curie point which was about 120.degree. C. In order to achieve the necessary increase, it has been proposed to form a solid solution of barium titanate and lead titanate which is prepared by substituting lead for part of the barium in barium titanate. In this manner it has been possible to increase the Curie point, however, the vaporization of lead during the sintering of the ceramics is great enough to cause disadvantages, in particular, it has been difficult to obtain ceramics having high densities and high strengths. The ceramics prepared have inferior electrical characteristics such as high voltage dependency, relatively low breakdown voltages and low reliability when used as heating elements. Moreover, the ceramics exhibit poor ageing variation under load at high temperature and have low durability which limits the utility of these ceramics when used as heating elements.
Accordingly, there exists a need for semiconductor ceramic compositions suitable for use as heating elements and which possess good electrical and physical properties.